Located north of the head of Orion and south of the horns of Taurus is a large open cluster known as Collinder 65 (CR 65), which is unlabeled on some star charts. You’ll find it about five degrees north of Lambda (λ) Orionis, stretched out across the Orion-Taurus border. (Lambda (λ) Orionis, a beautiful multiple star better known as Meissa, is at the center of another Collinder open cluster, CR 69).
The diameter of CR 65 varies, ranging from 140’ (2.3°) to 220’ (3.7°), depending on which source you consult. At any rate, it’s noticeably larger than its Orion relative, CR 69.
Both clusters are enticing binocular objects and rival one another in aesthetic appeal. Whereas CR 69 is dominated by the beautiful cream-colored white light of Meissa, CR 65 is illuminated by a handful of fifth and sixth magnitude Flamsteed-numbered stars. Scattered within that steady starlight are half a dozen multiple stars beckoning to the yearning eyes of a double star connoisseur.
The image above is a close approximation of what you would see in an 8×50 finder. In order to find your way around and have a sense of scale, it helps to know the distances between the various points, as well as the direction from star to star. Those morsels of information were invaluable later when I navigated from one star to the next while looking through an eyepiece. Below is the same image as above, but with the distances and directions added:
For use at the eyepiece of a refractor or SCT, here’s a mirror-reversed image of the same scene:
We’ll start with Σ 730, which is located at the center of the eastern edge of CR 65.
Σ 730 HIP: 25950 SAO: 94630
(AB is also H III 93, H N 124, and Sh 58; Aa, Ab is OCC 999)
RA: 05h 32.2m Dec: +17° 03’
Magnitudes Aa, Ab: 7.30, 7.30 AB: 6.06, 6.44
Separations Aa, Ab: 0.10” AB: 9.60”
Position Angles Aa, Ab: ????? AB: 141° (WDS 2013)
Distance: 2991 Light Years (Simbad)
Spectral Classifications: “A” and “B are both B7
This is a pair of stars that received attention from all the well-known double stars observers of the late 18th and early 19th centuries: William Herschel, John Herschel, James South, and F.G.W. Struve. It began with William Herschel seemingly confusing this pair of stars with 117 Tauri (source, scroll down to sixth title):
His position angle (which translates to 142° 27’) and separation for the two stars are reasonably close . However, his identification of this pair as 117 Tauri is an error which was discovered in 1821 by John Herschel and James South when they went in search of 117 Tauri and found it was a single star. Their discussion below is very clear and easy to follow (source, scroll down to last title):
If you read the quotes from Herschel’s observing notes carefully, which are just past the mid-point of the page, it’s clear that William Herschel was using 117 Tauri as a reference point to reach the star now identified as Σ 730. Herschel frequently included a Flamsteed-numbered star as a reference point in the first line of his observations, but in this case he left out the distance between 117 Tauri and the double star he observed in the published version, which led to John Herschel’s and James South’s search for it. Fortunately, they had access to William Herschel’s notes.
Prior to the John Herschel-James South observation, William Herschel apparently observed this pair of stars a second time in 1800 and cataloged it again as H N 124 (source):
That observation is rather difficult to follow. Apart from the coordinates, which match reasonably closely with those of Σ 730, his magnitudes of 9.9 don’t match at all, and I haven’t yet figured out what the Orion reference in the second line refers to. I’m not all that convinced the pair of stars he described in 1800 is the same pair he saw in 1782. Nevertheless, H N 124 is treated as a duplicate of H III 93 by S.W. Burnham in his 1906 catalog, and the WDS refers to H N 124 in its notes on Σ 730.
Next, we’ll traverse the middle of CR 65 in search of S 478, which is on the west side of this cluster at a distance of 1° 52’. You can see by the 280° position angle I included on the last chart (here’s the erect image, and the mirror image) that we’re going to move almost due west with a very slight inclination toward the north. Using 5.77 magnitude 117 Tauri as a stepping stone should get you to S 478 with little problem.
S 478 HIP: 25278 SAO: 94526
(AB is also H V 110; AC is WNO 52)
RA: 05h 24.4m Dec: +17° 23’
Magnitudes AB: 5.06, 8.79 AC: 5.06, 7.88
Separations AB: 106.70” AC: 705.20”
Position Angles AB: 271° (WDS 2011) AC: 252° (WDS 2010)
Distance: “A” is 46.9 Light Years, “C” is 45.9 LY (Simbad)
Spectral Classifications: “A” is F8, “B” is K0, “C” is K4
The AB pair was measured by Sir William Herschel during his observation of November 13th, 1782 (source, scroll down to sixth title):
His Flamsteed number, 111 Tauri, is correct in this case, but the separation he recorded is off considerably, which was noticed by James South when he observed the AB pair on January 17th and February 2nd, 1825 (source):
On the last line of his observation, South refers to the minor change in position angle of the primary and secondary as being too small to account for the large difference in his and Herschel’s separation measures. Those position angles translate to 273° 48’ for Herschel and 271° 17’ for South, and they’re the first hint of significant motion taking place in one of the two stars.
As it turns out, the primary has a rather high rate of proper motion, which is partly attributable to its relatively close distance of 46.9 light years from us. The “C” component, cataloged in 1897 as WNO 52, has a very similar rate of proper motion, and is located at a comparable distance, 45.9 light years. The Aladin photo below shows the rates of proper motion of all three of the S 478 components:
The similar motions of “A” and “C” are obvious, which in combination with their almost identical distances from us, has led to the conclusion the probability of their being physically related is close to 100%. The AB pair, on the other hand, is an optical pair, which is evident from the different rates of motion and direction of “A” and “B”.
The “C” component was added to S 478 in 1897 by the Washington Naval Observatory, which is the source of the WNO identifier assigned to AC. Also, there’s an eleventh magnitude star (TYC 1300-355-1) located 32” from “C” at a PA of 136°, for which there is no proper motion data, so it may or may not be moving in tandem with S 478 “A” and “C”.
You may have also noticed on the image above that S 478 “A” is labeled “BY Dra”, which is a reference to a class of variable star. The AAVSO (American Association of Variable Star Observers) designation for S 478 “A” (or 111 Tauri) is V1119 Tau. Their data on the star shows a magnitude range of 4.98 to 5.02, so don’t hold your breath in anticipation of a wild swing in brightness. The WDS data (shown above at right) also includes a note that S 478 “A” is a spectroscopic binary, which may account for its slight change in magnitude level.
So as you can see, hidden behind the visual appeal of the three stars of S 478, there are several layers of intriguing detail.
To get to our third stellar destination, Σ 697, we’ll move south and very slightly west a distance of 42.5’ to 6.1 magnitude 110 Tauri, and then continue due south another 39’ to our goal. (Here’s the erect image again, and the mirror image).
Σ 697 HIP: 25207 SAO: 94512
RA: 05h 23.5m Dec: +16° 02’
|STF 697||AB:||7.27, 8.10||25.90″||286°||2015|
|WAL 38||AC:||7.27, 10.83||97.90″||284°||2012|
|SMR 3||AD:||7.27, 10.07||249.30″||285°||2012|
|SMR 27||AE:*||7.27, 12.00||163.00″||290°||2012|
Distance: 874 Light Years (Simbad)
Spectral Classifications: “A” is B8, “B” is A, “D” is B9
Note: The magnitude of E has been changed to 13.89; see comment below in red.
When I first looked at the data on Σ 697, I was immediately intrigued by the similar position angles of all five of the components. And even though I was prepared for what I saw, I was still impressed by the sheer unlikely beauty of the configuration. Even 7.50 magnitude SAO 94498 and 8.53 magnitude HIP 25287 managed to line up with the components of Σ 697:
I had a suspicion the 10.07 magnitude shown in the WDS for “E” was too bright, so I pulled up an Aladin photo in order to first make sure the star existed. What I found was a reddish-orange star, which partially explained why it was difficult to see. I never was able to come up with a spectral classification for that star, but I did find magnitudes for it in both the UCAC4 and the Nomad-1 catalogs. UCAC4 shows a visual magnitude of 13.885 for “E”, and Nomad-1 lists it at a visual of 13.710, which explains why I couldn’t detect any sign of it in my five inch refractor. That component is a candidate for a magnitude change, so I’ll be in contact with Bill Hartkopf at the WDS. (NOTE: The magnitude of “E” has been changed to 13.89 as of 2/13/2015 and the UCAC4 proper motion data has also been added).
The AB pair, which seems to have eluded William Herschel, was discovered in 1828 by F.G.W. Struve, as shown at the left (source). As the excerpt shows, there’s been little change in the position angle and separation since 1828, which matches well with Simbad’s proper motion data on the two stars: -001 -008 (.001”/yr west, .008”/yr south) for the primary, and -003 -004 (.003”/yr west, .004”/yr south) for the secondary. In fact, there’s little movement among all four of the stars for which proper motion numbers exist, but what little motion there is suggests none of them are related physically to one another:
We’ll cover the last three multiple stars of CR 65 in the next post, along with a bonus that wasn’t on my list. Hopefully we’ll have some good seeing — we’re going to need it for this group!
Clear Skies! 😎